A first known method consists in generating eddy currents by means of a single coil carrying an alternating current, measuring the impedance of this coil and comparing it with a reference impedance value determined from a part considered to be good. This method suffers from the difficulties of requiring a preliminary determination of a reference impedance and of yielding a result which depends on the temperatures of the part and the coil.
A second known method is a differential method overcoming these difficulties. It consists in generating eddy currents in two adjacent locations in the same part, by means of two identical coils carrying the same current, one in phase, one antiphase, and in measuring the difference between the impedances of these two coils. This difference is zero when the two locations of the test piece have the same composition, independently of variation in temperature and without preliminary calibration. The difference in impedance is not zero if the composition of the part is not identical at the two locations where the coils are disposed.
These non-destructive test methods allow testing to a depth of less than or equal to 1 cm. This is a function of the frequency of the alternating current passing through the coil or coils. This frequency is at present between 10 Hz and 5 MHz. In order to discriminate small differences in the composition of a material, it is known to effect several measurements of impedance or differences between impedances at different frequencies. Each set of values thus obtained constitutes a signature of the slight difference in composition. A part is judged good or bad by comparing its signature with a reference signature.
A conventional non-destructive eddy current testing device comprises a Wien bridge generator providing a sinusoidal signal; a power amplifier receiving and linearly amplifying this sinusoidal signal to provide a power excitation signal; two coils connected by cables to the output of the power amplifier so as to be fed with the power excitation signal inphase or antiphase, depending on the sense of connection of the coils, which are electromagnetically coupled to the test piece; measuring means for detecting variation in the impedance of a coil; and digital processing means adapted in particular to store the results of the tests.
The excitation current passing through the coils is sinusoidal because the impedance of a coil is a magnitude which is only defined for a given frequency and because an impedance is measured conventionally by subjecting it to a sinusoidal current. Moreover a sinusoidal current has the advantage that it is not distorted by propagation along a cable of great length. In fact it is necessary in some applications to connect the coils to the control device by cables of great length, up to two hundred meters. It is well known that a line of great length has a dispersive action on the different frequencies making up a signal propagating on the line. A non-sinusoidal signal is thus distorted, in a manner which is a function of the characteristics of the line. This distortion of non-sinusoidal signals compromises the accuracy of the measurements of impedance or the difference between impedances, because the measuring method requires a sinusoidal signal with low distortion. The conventional solution thus consists in using a sinusoidal signal generator.
The generator itself should have a very low distortion so as not to compromise the accuracy of measurements. For improved measuring accuracy, it should also have excellent frequency stability, because any fluctuation in the frequency degrades the signal-to-noise ratio of the measurement and thus degrades its accuracy. This degradation hinders the detection of small defects in a part.
The generator used conventionally to provide a signal of given frequency is a Wien bridge oscillator, because it has adequate frequency stability and low distortion. Unfortunately a Wien bridge oscillator does not exhibit these qualities when its frequency is varied, which makes it difficult to implement a variable frequency generator and reduces the accuracy of the measurements.
It is known to implement the measuring means with two synchronous detectors. In this case the accuracy of the measurement is reduced if the length of the cables is great. Thus the excitation signal takes a certain time to reach the coils and the measurement signal takes the same time to reach the synchronous detectors from the coils. Means are associated with the Wien bridge to provide the two synchronous detectors with respective sinusoidal reference signals of the same frequency as the excitation signal and in quadrature. These means are conventionally located near to the synchronous detectors, whereby the reference signals are phase-advanced relative to the measurement signal and the synchronous detectors function under non-optimal conditions. This phase advance is a function of the length of the cables and thus varies, depending on the circumstances of use of the control device; it is difficult to compensate since it is variable and it is necessary to act on an analog signal.
The object of the invention is to provide a non-destructive eddy current testing device which allows the frequency to be varied over a large range while obtaining an accuracy of measurement at least as great as that obtained at a fixed frequency with a Wien bridge oscillator, and which avoids accuracy being reduced on account of the use of cables of great length.